The invention relates to an optical fiber amplifier which directly amplifies an optical signal.
The invention especially applies to an erbium (Er3+)-doped fiber amplifier (EDFA) which utilizes an erbium-doped optical fiber (EDF). In a conventional EDFA, one end of the EDF is connected through an optical isolator to an input and the other end is connected through an optical isolator to an output. A pumping illumination source provides pumping illumination for the EDF through a photocoupler. Then, the pumping illumination is absorbed by the EDF sufficiently raise the population inversion.
When an input light signal is introduced into the excited EDF from the input-side of the optical fiber, the input light signal is gradually amplified by a stimulated emission of the EDF and the amplified input light signal is output to the output-side of the optical fiber.
Here, the wavelength of the light signal is selected on bands 1550xcx9c1560 nm whose loss in a silica fiber is the smallest. Moreover, a wavelength of the pumping illumination is selected on bands 980 nm or 1480 nm which are absorption bands of the EDF. The EDFA has an amplification region on bands 1530xcx9c1560 nm, and the EDFA is effective for narrow band light signal transmission by single wavelength. But, in order to utilize the effectiveness an optical fiber transmission line, a wavelength-divisional-multiplex(WDM) system which transmits multiple wavelength channels together is desirable.
When the EDFA is applied to the WDM system, it is important to improve the wavelength-to-gain-deviation characteristic(: gain equalizing of wavelengths), especially since a signal power deviation between each wavelength after transmission becomes fairly large, unless the gain of each wavelength channel at every EDFA repeater is substantially equal. As a result, as the number of repeaters increases, transmission distance is limited by the S/N ratio of the wavelength signal which has minimum amplitude.
For this reason, when a scattering of signal power and S/N ratio between wavelength channels is compensated, the gain equalization of the EDFA is an indispensable technique. Various gain equalizing techniques of the EDFA have been proposed.
For example, it is known that gain dependence of wavelength may be equalized by an optical filter; gain characteristics of the EDF may be controlled by doped aluminum (Al) concentrations, and the EDF may be cooled toward a low temperature to effectuate gain equalization. These techniques allow gain equalizing to be achieved at bands 1540xcx9c1560 nm, but not over broader bands. Moreover, in practice, a distance between repeaters is not fixed nor is the light signal power input to the EDFA. These points also make gain equalization of the broader bands difficult.
So, it is desired to provide an optical fiber amplifier which can realize gain equalization over a broader input signal power and wavelength bands.
In carrying out the invention in one preferred mode, an optical fiber amplifier comprises a optical fiber for direct amplifying a light signal, an pumping illumination source for the optical fiber for providing pumping illumination, detecting means for detecting an input light signal, gain equalizing for equalizing a gain of the light signal and a gain equalizing means controller for controlling the gain equalizing means based on the detected input light signal which is detected by the detecting means, so that the optical fiber amplifier can realize gain equalization over a broader input signal power and wavelength bands.